The present invention relates to a coating apparatus and method. More particularly, the invention relates to a method and apparatus for coating a substrate in which, in order to manufacture photographic photosensitive materials such as photographic film and photographic paper or magnetic recording materials such as magnetic recording tapes, photographic emulsion or magnetic solution is applied to a long belt-like substrate (hereinafter referred to as a web).
Methods with which various coating composition are applied to supports of plastic film or paper and are then dried to manufacture desired products are extensively employed in a variety of manufacturing applications, such as in the manufacture of photographic film or paper, magnetic recording materials, pressure-sensitive recording sheets, and heat-sensitive recording sheets.
An example of a method which is extensively utilized for applying a coating composition is a method using a multilayer slide bead coating device as proposed by Russell et al. in U.S. Pat. No. 2,761,791. In this method, a plurality of coating compositions flowing down slide surfaces are caused to form a bead at a position below the slide surface where the coating compositions meet the web, which is continuously conveyed, so that the coating operation is achieved with the aid of the bead.
In such a conventional coating device, as shown in FIG. 1, a coating head 3 is disposed adjacent to a web 2 which is conveyed while being guided by a coating backing roll 1. Metering pumps 8 and 9 supply coating compositions 10 and 11 through main supply pipes 6 and 7 into cavities 4 and 5 in the coating head 3, respectively. The coating compositions thus supplied are extruded in the form of ribbons from slots 12 and 13, thus flowing, as two layers, down a slide surface. The two layers flow down the slide surfaces without mixing to a bead section 14 where the two layers are applied to the web simultaneously. A pressure reducing chamber 15 is connected to a vacuum pump 16, thus serving as a bead stabilizing back pressure supply.
In a coating method such as the above-described slide bead coating method, an extrusion bead coating method, a hopper coating method, or a curtain coating method in which a coating composition is supplied through a main supply pipe into a cavity formed in the coating head and is then extruded through a slot communicating with the cavity for application to the web, the uniformity of coating in the lateral direction of the web depends on the uniformity of the slot. Accordingly, in order to obtain satisfactory coating results, it is essential to form the slot with a very high accuracy.
Another serious drawback is that the residue of the coating composition in the slot and the cavity has a tendency to solidify, thus lowering the uniformity of coating in the lateral direction of the web. Moreover, the residue can flake off and be coated on the web, as a result of which the quality of the coated product is greatly lowered. It is considerably difficult to remove the residue of the coating composition through the slot, and accordingly it takes a relatively long period of time for cleaning the coating device.
This phenomenon is attributed to the fact that, when coating is carried out for long periods, some amounts of the coating composition are stagnated in the cavity of the coating head. Specifically, in the case of photographic emulsion, a gelatin bridge of a hardening agent, whose viscosity increases with time, tends to be created. Also, in the case of magnetic solutions, a condensate of a dispersing agent is created, and the material thus created accumulates in the cavity or sticks to the slot.
Typically, a metering pump is operated to supply the coating composition through a main supply pipe to the cavity in the coating head, and the coating composition thus supplied is passed to the slot, which governs the uniformity of coating in the lateral direction of the web. Accordingly, the coating composition is abruptly spread over the entire width of the coating head, and therefore the coating composition is liable to be stagnated at both ends, in the lateral direction, of the cavity. It is considerably difficult to design the cavity so that, in applying a coating composition having certain properties under certain coating conditions, stagnation of the coating composition is entirely eliminated. Especially in the case of a general-purpose coating head, stagnation of the coating composition occurs unavoidably.